1. Field of the Invention
This invention pertains to the field of isocyanate synthesis. More specifically, it discloses a non-phosgene process for the preparation of 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate.
2. Description of the Prior Art
3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate, subsequently referred to also as isophorone diisocyanate or abbreviated as IPDI, is currently prepared solely through the phosgenation of 3-aminomethyl-3,5,5-trimethylcyclohexylamine, subsequently referred to also as isophorone diamine or abbreviated as IPDA, and the subsequent thermal cleavage of the intermediately formed isophorone dicarbamyl chloride into IPDI and hydrogen chloride.
The problems with this process are the high conversion of chlorine via phosgene and carbamyl chloride into hydrogen chloride, the toxicity of the phosgene and the expensive safety problems associated therewith, the corrosiveness of the reaction mixture, and the instability of the solvents that are generally used.
Thus, there have been numerous attempts to prepare isocyanates, primarily aromatic di- and/or polyisocyanates without use of phosgene.
In EP-A No. 28 338 aromatic di- and/or polyisocyanates are prepared in a two-step process, whereby primary aromatic di- and/or polyamines are reacted in the first step with O-alkylcarbamide acid esters in the presence or absence of catalysts and, in some cases, urea and alcohol to form aryldi- and/or polyurethanes, and the ammonia formed in this process may, in some cases, be separated off, and the aryldi- and/or polyurethanes that were obtained can be converted into aromatic di- and/or polyisocyanates in the second reaction step by means of thermal cleavage. In this way, aromatic di- and/or polyisocyanates can be prepared with high yields and without using phosgene.
DE OS No. 31 08 990 describes the preparation of IPDI through the thermal cleavage under pressure of 3-ethoxycarbonylaminomethyl-3,5,5-trimethyl-1-ethoxycarbonylaminocyclohexa ne in the presence of dibenzyltoluene as a solvent and a catalyst mixture of toluene methylsulfonate and diphenyl tin dichloride. No information is given on obtaining the initial components, isolating and purifying the initial components, possible recovery of the solvent, or on the catalyst mixture. Thus, calculations of the economic feasibility of the process cannot be made.